Photovoltaic devices such as e.g., photovoltaic modules or cells can include semiconductor and other materials deposited over a substrate using various deposition systems and techniques. One example is the deposition of a semiconductor material such as cadmium sulfide (CdS) or cadmium telluride (CdTe) thin fihns over a glass substrate using a processing chamber such as e.g., a vapor transport deposition (VTD) chamber.
During the processing, it is important for a system controller to know the positions of the substrates within the processing chamber to ensure, among other things, that there is proper spacing between the substrates and to know what process the substrates are currently undergoing. Typically, the edge position of each substrate is checked before the substrate is placed into the chamber. This gives the controller an initial point to track the substrate as it progresses through the chamber. Unfortunately, the actual substrate position can be shifted/offset from the controller's calculated substrate position at different times during the process. For example, a position shift can occur as the substrate is entering the processing chamber due to the high speed transfer used to place the substrate into the chamber. Other shifts between the actual and controller calculated positions can occur due to speed changes arising from material build up on the rollers transporting the substrates.
Differences between the controller calculated substrate position and the actual substrate position can adversely impact the processing or lead to mishandling of the substrates. Thus, processing systems will incorporate edge detection mechanisms, portions of which reside within the chamber. Each detection mechanism includes a laser that emits a light beam through a chamber window to a reflector located within the chamber at a point somewhere along the substrate travelling path. The reflector reflects the beam back to a detector. When the substrate passes by, the beam is interrupted, signaling the presence of the substrate. The controller can use this information to try to compensate for the new substrate position.
The above detection mechanism has some shortcomings and relies on several factors to be successful, some of which cannot be controlled. For example, the chamber windows must be clean to allow the light beam to enter the chamber, be reflected back and detected. Keeping the windows clean will require additional maintenance and down time, which is undesirable. Moreover, the laser, reflector and detector must remain properly aligned, which is difficult to achieve due to the processing and vibrations within the chamber. Furthermore, there is the general need to prevent the light beam path from being blocked or disrupted by anything other than the substrate to prevent false detections, which is an onerous task.
Accordingly, there is a need and desire for a better way to detect a position of substrate that is being transported within a substrate processing chamber.